An optical navigation device, such as an optical mouse, typically includes an optical navigation sensor integrated circuit (IC) that functions as a miniature digital camera to continually collect images of a surface that the device is sitting on and to determine the speed and direction that the device is being moved across the surface by comparing sequentially recorded frames of image information. Image frames are collected at a very high rate, such as 1,500 image frames per second, and the resolution of the optical navigation sensor IC is high enough to detect very small movements of the device relative to the navigation surface.
With an optical navigation device such as an optical mouse, there is a need to recognize when the device has been lifted off the navigation surface. This is important because the act of lifting the device off the navigation surface indicates that the user wants navigation tracking to be temporarily suspended. Detecting lift off when navigating on an opaque surface is typically done by processing the image information that is generated by the navigation sensor IC.
Sometimes an optical navigation device is used on a transparent structure such as a glass plate that covers a desktop. One technique for enabling an optical navigation device to navigate on a transparent structure as well as an opaque structure involves eliminating the navigation device's focusing optics. While an optical navigation device can be configured to support navigation tracking on both transparent and opaque structures without focusing optics, elimination of the focusing optics renders conventional lift detection techniques ineffective over the distances required for practical lift detection, e.g., less than about 10 mm.
Therefore, what is needed is a lift detection technique that is effective on a navigation surface whether the navigation surface is on a transparent structure such as a glass plate or an opaque structure such as a desktop.